Doctor vs. The doctor

Why, why, why do medical assistants constantly refer to a doctor as “Doctor” as is “Doctor will see you now.” or “Doctor wants you to know . ..”   I’m certain that almost every other English noun referring to a person has an article in front of it. The doorbell rings, someone answers and yells, “THE plumber is here.”

“Room Service” may be here, but “THE waiter” rolls it in.

I’m guessing at the asylum, the workers say, “Here comes THE  new nutcase.” Yet, they tell the nutcase” DOCTOR will be right in.

Obviously, I am still irritable.